The Manatee County Fertilizer Ordinance requires that fertilizer applications take into account nutrients that may be supplied by reclaimed water irrigation. This article discusses the issue for professionals. The calculations can be complex and in most cases the quality of the reclaimed water can only be estimated.
What is reclaimed water?
Reclaimed water is the end product of wastewater treatment processes conducted at each of the county’s three (3) Water Reclamation Facilities (WRFs). A multi-stage treatment process eliminates pathogens (solids, organics, and viruses), but the reclaimed water still retains nitrogen, phosphorus, and other nutrients that work as fertilizers to enhance ornamental plant and turf grass growth. This process produces water not suitable for human or animal consumption, but ideal for irrigation purposes.
Manatee County Ordinance 11-21 (Section 2-35-13 of the Manatee County Code of Ordinances) recommends that the amount of fertilizer applied to property irrigated with reclaimed water be adjusted to account for the nutrients contained in the reclaimed water. The amount of additional fertilizer needed depends on the concentration of nutrients contained in the reclaimed water, the amount of reclaimed water irrigated with, and the plant fertilized.
What is the nutrient content of reclaimed water?
In 2006, Manatee County completed construction of the MARS (Manatee Agricultural Reuse System) which connects the reclaimed water distribution networks of all three WRFs. The MARS allows the redistribution of reclaimed water throughout the county based on demand. Because of this redistribution, nutrient concentrations in reclaimed water will vary depending on WRF origin, demand, and season.
The following table illustrates average seasonal Total Nitrogen (TN) and Total Phosphorus (TP) concentrations contained in the county’s reclaimed water, determined utilizing weekly analytical data from each of the county’s three WRFs:
Table 1. Average Nutrient Concentrations in Reclaimed Water
|
Nutrient
|
Dry Season
(Oct – May)
|
Wet Season
(Jun - Sep)
|
|
Avg Conc
|
Range
|
Avg Conc
|
Range
|
|
Total Nitrogen
|
7 mg/L*
|
2 – 18 mg/L
|
8 mg/L
|
3 – 15 mg/L
|
|
Total Phosphorus
|
2 mg/L
|
0.1 – 5 mg/L
|
2 mg/L
|
0.7 – 5 mg/L
|
* Milligrams per liter (mg/L).
Manatee County Ordinance 11-21 requires soil testing to demonstrate a need for the application of phosphorus containing fertilizers, primarily because phosphorus is naturally abundant in the soils of Manatee County. Adding to that phosphorus from reclaimed water irrigation, there will be seldom instances where the addition of phosphorus fertilizer is needed. Consequently, the following discussion of nutrients added by the use of reclaimed water irrigation will be restricted to nitrogen.
How do I determine the amount of nutrients I’m applying with reclaimed water?
The amount of nutrients applied with each reclaimed water irrigation event depends on both the concentration of nutrient in the reclaimed water (Table 1), and the amount of irrigation water applied during the irrigation event. A rain gauge, or similar device, placed in the area irrigated will provide a general estimate of the amount, in inches, applied to the irrigated area. If micro-irrigation, seepage hoses, or other irrigation method is utilized, volume determination will require a flow gauge on the inlet to the irrigation system or zone.
Mathematical calculations are needed to convert nutrient concentrations and volume of irrigation into pounds of nutrients applied. To simplify, Table 2 below provides the amount of nitrogen applied, in lbs/1000 ft2, when irrigating with reclaimed water under various amounts and frequencies:
Table 2. Total Nitrogen (TN; lbs/1000 ft2/yr) Applied When Irrigating with Reclaimed Water1
|
Frequency:
|
|
|
|
Irrigation Amount
|
|
|
|
|
|
|
|
0.25"
|
0.5"
|
0.75"
|
1"
|
1.25"
|
1.5"
|
1.75"
|
2"
|
2.25"
|
2.5"
|
2.75"
|
3"
|
|
Once/2 Months
|
0.1
|
0.1
|
0.2
|
0.2
|
0.3
|
0.3
|
0.4
|
0.4
|
0.5
|
0.5
|
0.6
|
0.7
|
|
Once/Month
|
0.1
|
0.2
|
0.3
|
0.4
|
0.5
|
0.7
|
0.8
|
0.9
|
1.0
|
1.1
|
1.2
|
1.3
|
|
Once/Week
|
0.5
|
0.9
|
1.4
|
1.9
|
2.4
|
2.8
|
3.3
|
3.8
|
*
|
*
|
*
|
*
|
|
Twice/Week
|
0.9
|
1.9
|
2.8
|
3.8
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
|
3 Times/Week
|
1.4
|
2.8
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
|
Daily
|
3.3
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
* Amount exceeds maximum annual allowance of 4 lbs TN/1000 ft2 under Ordinance 11-21
1 Based on TN content of 7 mg/L (avg content for dry season from all three WRFs)
To determine the amount of nitrogen applied when irrigating with reclaimed water, simply cross-reference the irrigation amount, shown along the top of Table 2, with the frequency of irrigation, shown at the left side of the table. The resulting number is the amount of nitrogen applied per 1000 ft2 per year. For example, if you are irrigating once a week and applying ½ inch of reclaimed water during each irrigation event, you are applying 1.9 lbs of nitrogen per 1000 ft2 per year*. Typical turf nutrient needs in this region of Florida are approximately 4 lbs TN/1000 ft2 per year. As can be seen in Table 2, irrigation with reclaimed water can provide a substantial amount of the nitrogen needed by most turf types. Depending on irrigation frequency and plant type, additional fertilizer may not be necessary for healthy plant growth!
*The amounts shown in Table 2 are based on the average concentration of nitrogen during the dry season, when irrigation is most common. Adjustments in the amounts applied would be necessary for irrigation during the wet season.